The present invention relates to a lens driving apparatus used in an optical apparatus such as a video camera. The lens driving apparatus moves a magnification-varying lens and moves a focus lens in order to compensate for image plane variation caused by movement of the magnification-varying lens.
Lens driving apparatuses used in the optical apparatus include one provided with a stepping motor as a driving source and a driving mechanism constituted by a lead screw rotated by the stepping motor and a rack attached to a lens holding member holding a lens and engaging with the lead screw.
For miniaturization of the optical apparatus, miniaturization of a battery thereof, which is achieved by lowering a rotation speed and an output torque of the stepping motor to reduce power consumption, is effective.
However, lowering the output torque of the stepping motor makes it easy to cause step-out of the stepping motor when a lens driving load increases. Since the driving amount of the stepping motor is generally controlled based on the number of pulses of a driving pulse signal applied to the stepping motor, the step-out collapses the relationship between the driving amount of the stepping motor and the position of the lens (lens position), which makes it impossible to control the lens position accurately. It is also possible to lower an engagement pressure (backlash reducing pressure) of the rack with respect to the lead screw in order to reduce the lens driving load. However, such lowering of the engagement pressure makes it easy to cause a situation in which the rack skips a tooth of the lead screw, that is, so-called “tooth skip” due to impact, which makes it impossible to control the lens position accurately.
Japanese Patent Laid-Open No. 05-281449 discloses a lens driving apparatus including a position sensor which detects the position of a lens that is moved by a stepping motor. In this apparatus, when the step-out of the stepping motor or the tooth skip of a driving mechanism occurs, correction of control of the stepping motor according to the lens position detected by the position sensor is performed to enable accurate lens position control.
Further, Japanese Patent Laid-Open No. 11-110045 discloses a lens driving apparatus capable of improving accuracy of lens position control, apparatus which detects the position of a lens that is moved by a stepping motor to perform feedback control of the stepping motor based on the detected lens position.
Optical apparatuses include one having a zoom optical system capable of zooming as an image taking optical system. In particular in video cameras, a so-called rear focus type zoom optical system is often used. In the rear focus type zoom optical system, variation of magnification performed by movement of a magnification-varying lens causes image plane variation (in other words, defocusing). Therefore, compensation (correction) for the image plane variation is performed by moving a focus lens that is disposed closer to an image plane than the magnification-varying lens to maintain an in-focus state. The position to which the focus lens is moved is determined in advance according to electronic cam data (hereinafter simply referred to as “cam data”) showing a relationship between a position of the magnification-varying lens and a position of the focus lens at each object distance.
In order to perform the variation of magnification while maintaining the in-focus state in such a zoom optical system, it is necessary that the positions of the magnification-varying lens and focus lens accurately keep the relationship determined by the came data. The position of the magnification-varying lens can be obtained by counting the number of pulses of the driving pulse signal applied to the stepping motor that moves the magnification-varying lens from a predetermined reference position. Therefore, control of the focus lens position based on the number of pulses of the driving pulse signal (hereinafter referred to as “driving pulse number”) may move the magnification-varying lens and the focus lens while keeping the relationship determined according to the cam data.
However, an engagement portion of the lead screw and the rack of the driving mechanism that transmits driving force of the stepping motor to the magnification-varying lens and a connecting portion of a lens holding member holding the magnification-varying lens and the rack respectively include a backlash. Moreover, the lead screw and the rack include a manufacturing error such as a screw pitch error. In addition, the stepping motor includes an individual variability in an actual driving amount with respect to the driving pulse number. Such a backlash, a manufacturing error and an individual variability cause mismatch between the driving pulse number applied to the stepping motor and the actual position of the magnification-varying lens, which deviates the positions of the magnification-varying lens and focus lens from the relationship determined according to the cam data. In other words, defocusing is caused with the variation of magnification.
Japanese Patent Laid-Open Nos. 05-281449 and 11-110045 only disclose that the position sensor is provided for the magnification-varying lens moved by the stepping motor to improve the position control accuracy of the magnification-varying lens.